I need to book a last-minute reward and just realized that the miles from one transaction done over a month ago still haven't posted - leaving me 800 miles short of my reward.

I know of various ways to get miles posted within 3-4 days (Payback, transfer from various hotel programs etc.), but I'm wondering: Is there any method where miles post (almost) instantly?

(If M&M still offered the option to purchase miles, I'd just buy 1k miles; but that seems to have been discontinued).

Thanks in advance!